1. Field of the Invention
The present invention relates to oscillation circuits incorporated in large scale integrate circuits (LSIs) and contemplates devices capable of changing their oscillation frequency by voltage control over a wide range and continuously. Thus the present oscillation circuit can be utilized as LSI""s reference clock oscillators, local oscillators for frequency modulation and demodulation, voltage-controlled oscillators (VCOs) for phase locked loop (PLL), and other similar, various types of oscillators, and as it allows an oscillation frequency to be dynamically and continuously controlled, it can contribute to enhanced performance of LSI.
2. Description of the Background Art
A circuit configuring an LSI generally requires a VCO in a PLL, a local oscillator in a radio frequency (RF) modulation and demodulation circuit, or other similar, various types of oscillation circuits. In an LSI, however, it is difficult to introduce large inductance (L) and large capacitance (C) in high integration. As such, it has been difficult to incorporate an LC oscillator of a wide range of frequency (intermediate and low frequencies in particular). Conventional LSIs requires that an element having large inductance, large capacitance or the like, a crystal oscillator, or the like be connected externally.
Furthermore, it has also been difficult to electrically, continuously modulate oscillation frequencies of conventional oscillation circuits. An oscillation frequency can be readily modulated by a divider if it is modulated discretely (digitally),i.e., in accordance with a power of 2. Continuous (analog) modulation required for example for VCOs, however, is difficult to obtain, since it requires that the oscillation circuit""s resonance parameter or the like be modulated. Conventionally, an oscillation frequency is continuously modulated for example by using a varicap capable of modulating a capacitance with a voltage, or by controlling a current of a feedback circuit. Furthermore, another, proposed approach to electrically modulate an oscillation frequency is employing a backgate bias voltage of a ring oscillator to modulate a threshold value of a transistor. These approaches, however, suffer small frequency modulation ranges.
FIG. 3 shows a basic circuit configuration of a ring oscillator. The ring oscillator is characterized in that inverters 100 in an odd number of stages (a logic inversion circuit) connected in series form a delay circuit 105 outputting a signal which is connected to an input of delay circuit 105 to form a feedback loop of a delay signal. It is referred to as a ring oscillator because the odd number of inverters 100 are connected in a ring. One of outputs of the inverters is provided as an output of the ring oscillator via a buffer 101. Each inverter 100 is typically configured by a CMOS (Complementary Metal Oxide Semiconductor) inverter circuit shown in FIG. 4. A ring oscillator has its oscillation frequency determined by its inverter""s driving ability and load capacitance and the number of the inverters connected in stages. If each inverter delays a signal by a time Td, then an oscillation frequency f is:
f=1/(2xcexa3Td)≈1/(2n*Td)xe2x80x83xe2x80x83(1), 
wherein xcexa3 represents the sum with respect to all of the inverters connected in the ring and n represents the number of the stages of the inverter ring. As such, for a ring oscillator, oscillation frequency f can be set by adjusting an MOS transistor""s gate width to gate length ratio to change delay time Td at each inverter or by changing number n of inverters connected in stages. In other words, for the conventional ring oscillator shown in FIGS. 3 and 4, if power supply voltage is not changed, an oscillation frequency cannot be changed as long as the layout (the geometry of hardware) is not changed.
An exemplary configuration of a frequency variable ring oscillator oscillation circuit will now be described. FIG. 5 exemplarily shows a configuration of a circuit of a ring oscillator capable of modulating an oscillation frequency with a voltage. The FIG. 3 example is distinguished from the FIG. 5 conventional example only in that an MOS transistor""s backgate voltage (or well potential) is not connected to a power supply voltage GND or Vd but to a control terminal receiving a voltage Vwn or Vwp independent of the power supply voltage. The MOS transistor can have a threshold value modulated by its backgate voltage. As such, by changing voltages Vwn and Vwp providing each backgate voltage, the MOS transistor""s threshold value can be changed, the transistor""s drive current can be modulated, and each inverter""s delay time Td can be changed. Consequently, oscillation frequency f represented by expression (1) can be modulated.
In the FIG. 5 configuration, as a backgate voltage varies, frequency modulation characteristics are obtained, as shown in FIG. 6. In FIG. 6, xcex94Vw represents an amount of modulation of voltage Vwn and Vwp, and Vwn=Vd+xcex94Vw and Vwp=GNDxe2x88x92xcex94Vw for the sake of illustration. Furthermore, xcex94f indicates a ratio of a frequency after modulation to that before modulation (i.e., xcex94Vw=0). In this exemplary, prior art configuration, despite a backgate voltage""s modulation amplitude has been changed to correspond to a power supply voltage""s amplitude, a frequency modulation of approximately 30% is only achieved. This is attributed to the fact that the transistor has a threshold value in proportion to a square root of the backgate voltage. It is apparent that in this exemplary, prior art circuit configuration, there is a limit in modulating an oscillation frequency over a wide range.
The present invention provides an oscillation circuit including a plurality of inverters configuring a ring oscillator, at least one of the plurality of inverters being configured of an MOS transistor capable of modulating a gain coefficient electrically in analog manner.
The present invention in another configuration provides an oscillation circuit including N inverters connected in series to configure a ring oscillator, N being an odd integer of no less than three, at least one of the N inverters including an MOS transistor capable of modulating a gain coefficient electrically in analog manner.
Preferably the N inverters each include an MOS transistor of a first conductivity having a first control gate receiving a first voltage to allow the gain coefficient to continuously vary in accordance with the first voltage, and an MOS transistor of a second conductivity opposite to the first conductivity, having a second control gate receiving a second voltage to allow the gain coefficient to continuously vary in accordance with the second voltage.
More preferably the N inverters are each driven by first and second power supply voltages distinguished from the first and second voltages and the MOS transistors of the first and second conductivities receive one and the other of backgate voltages, respectively, set to be the first and second power supply voltages, respectively.
Alternatively, more preferably, each MOS transistor of the first conductivity has a source, a drain and a normal MOS gate and in addition the first control gate for controlling a direction of an electric field in a channel region, and each MOS transistor of the second conductivity has a source, a drain and a normal MOS gate and in addition the second control gate for controlling a direction of an electric field in a channel region.
Alternatively, more preferably, the oscillation circuit further includes a buffer for outputting a signal output from one of the N inverters.
Therefore a main advantage of the present invention is that an MOS transistor capable of continuously modulating a gain coefficient xcex2 can be used to configure a ring oscillator to implement an oscillation circuit having a capability of a wide frequency modulation. As such, when the present oscillation circuit is used as a source of a reference clock of an LSI, the LSI""s operating frequency can dynamically be adjusted to allow fine control of low power consumption.
Furthermore, it can also be used in a variety of fields as an LSI incorporated circuit represented for example by VCOs for PLL and local oscillation circuits for RF signal modulation and demodulation circuits as it has a simple circuit configuration and a compact circuit size.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.